Drawing up a budget for the garden office

I have no real experience with budgets. I simply made a list of every item I thought I’d need, divided into the phases, with a generous markup for price rises and miscalculations. I also estimated quantities generously. It gives an indication of what the building is going to cost.

For the renovation of our ground floor I also budgeted the contractor’s price +30%. I knew from the TV series that for 1,001 reasons you can’t foresee everything that might happen. From geopolitical games that drive up prices to a forgotten socket. In the end we went 10% over, and that was down to new insights and choices on my part, but also simply to the contractor not having everything on the quote. That last point sounds bad, but it’s actually the reality even with a good contractor. Extra work is standard, and if you push a contractor for all-in prices, you’ll simply see that risk come back in your quote as an extra risk margin.

Why no AI yet?

At this stage of the build I was already working with agentic development, but not yet outside IT. I’m certain my budget would be better if I asked Claude to go over it again now. I’m also certain I’d have had the build less clearly in my own head and wouldn’t have been able to ask follow-up questions.

What’s in the budget

The list is divided into eight parts. Each line has a unit, a quantity, a unit price and a link to the web shop where I got that price. After each subtotal the same amount appears again with 20% on top. This is my margin for price rises and miscalculations. The amounts below include that markup.

• Foundation — € 4,616. 16 m³ of concrete (strips of 25.2 × 0.40 × 0.70 plus a 40 m² floor slab), formwork of shuttering plywood and spruce battens, reinforcement mesh, sheeting and two hours of concrete pump.
• Timber frame — € 8,031. By far the largest item: 210 m of 38 × 140 spruce for the walls, 150 m of 38 × 184 for the roof, two steel beams, hydrophobised base blocks, 65 m² of wood-fibre insulation, 105 m² of OSB on the inside, wood-fibre board and vapour-open membrane on the outside, flashing, a kick board and a damp-proof course along the concrete.
• Roof — € 3,024. 40 m² of bitumen and 26 m of roof edge trim.
• Finishing — € 3,824. Corrugated steel sheet as cladding out of sight, plasterboard and cement board on the inside, and brick slips for the visible facade.
• Floor — € 4,158. 100 mm XPS, a 5 cm cement screed, PVC and the membrane underneath.
• Groundwork — € 804. Sewer pipe, buried cable, ducting and a water pipe towards the house. The smallest item on the whole list.
• Doors and frames — € 4,980. 30 m of frame timber, a bluestone sill, an external door and 4 m² of triple glazing.
• Electrics — € 1,080. Sockets, internet cable and the back boxes.

Altogether that’s € 25,431 bare and € 30,517 with the markup. Labour isn’t included, because I’m doing that myself. Nor are air conditioning, ventilation, sanitaryware and kitchen — those are separate items for later.

The most important figure is the last one. I went into the conversation with the architect with an estimate of 10 to 15 thousand euros for the build itself. The list says double that.

View the budget (PDF)

What now?

Fair’s fair, the price was a bit of a gulp, but the budget does make the work manageable again. The price will no doubt turn out higher still, and on top of that the landscape gardener and the garden plans aren’t included at all yet. Our advantage is that we’re in no hurry. It doesn’t matter if the garden has to wait another year or if the interior finish of the garden office isn’t done yet.

I then discussed the budget with my brother-in-law and asked him to make drawings and to ask a local structural engineer for a quote.